Present day photolithographic printing plates are conventionally formed of a mono- or multimetal base substrate having a coating of light sensitive, photochemically reactive material thereon. Such light sensitive, photochemically reactive materials employed in lithography, including presensitized polymer formulations will hereinafter be generally referred to and broadly identified as "photopolymer" or "photopolymeric" coatings or materials.
One widely used example thereof is the conventional bimetal plate formed of an aluminum alloy or stainless steel base substrate having a thin layer of copper on the surface thereof which, in turn, is overlaid by a photopolymer coating thereon. After exposure, the portions of the photopolymeric coating definitive of the non-image areas are removed and the thus exposed underlying copper surface etched away to expose the base substrate surface which is, if it is stainless steel for example, of pronounced water receptive or hydrophilic character. Following such copper removal in the non-image areas, the portions of the photopolymeric coating definitive of the image areas are then removed to expose the underlying oleophilic copper surface thereunder. Plates of this type are generally characterized by high print quality and relatively long press life and, despite their relatively expensive nature, have been extensively used in recent years.
Another and less expensive example thereof is the conventional monometal plate formed of an aluminum alloy base substrate having the surface thereof appropriately grained and anodized and overlaid with a photopolymeric coating. Such coating, which in this type plate is not entirely removed and portions of which ultimately will serve as the oleophilic image defining area of the finished and developed plate, can be either positive or negative working, depending upon its chemical nature. Plates of this type, although relatively less costly and offering simplified processing procedures, are generally characterized by shorter press life and higher frequency of print quality problems occasioned by the durability of the photopolymer surface, the adhesion of the photopolymer to the substrate, and the brittle nature and other characteristics of the anodized non-image defining surface areas on the plate.
The performance inadequacies of the above described monometal presensitized photopolymer type coated plates, sometimes called "surface" plates, together with the inherent more difficult processing requirements for the above described and more expensive bimetal type plate, as compounded by current EPA regulatory requirements relating to the disposition of heavy metals and corrosive etchants, has created a need for an improved photopolymer printing plate that offers the advantageous print quality and long press life characteristics of the bimetal plate with the advantageous simplified processing procedures of the monometal photopolymer coated type plate.
Recent developments in the art have demonstrated the commercial practicality of forming improved photopolymer coated printing plates of the second type described above, i.e. of the type where a developed photopolymer surface is definitive of the image area on a finished plate, by the direct deposition of chromium on a base metal substrate, such as aluminum alloy or steel, overlaid with a coating of either a positive or negative working photopolymer thereon. Normal processing of either of these plates, i.e. positive or negative working, yields a chromium surfaced non-image area which, after finishing, is highly hydrophilic in character. Such plates, as disclosed in copending application Ser. No. 134,636, filed Apr. 11, 1980, present an electrodeposited chromium surface having an improved secondary grain structure characterized by a close packed, cragged surface and a labyrinthine understructure compositely defining an electroplated layer of high surface area and porosity that provides for markedly better adhesion of the pre-exposed photopolymer coating thereon, improved water carrying characteristics in the processed plate, an increased press life because of the improved durability of the chromium surface, and an enhanced print quality on press. The high surface area and porosity characteristics of such electrodeposited chromium layer have somewhat restricted the selection of photopolymeric coatings that can be most effectively utilized thereon to those that can attain the required degree of adhesion to the underlying chromium layer and yet also permit, after exposure, easy and effective removal thereof from the non-image areas during the subsequent development process. While not fully understood at the present time, it is believed that a combination of the high surface area, porosity and labyrinthine understructure of the electrodeposited chromium layer results in the entrapment of minute amounts of photopolymer in the non-image areas of the developed plate which causes tinting or scumming sensitivity on press. Performance data obtained to date indicates a markedly improved press life and print quality, but with further increases in press life being limited by scumming or tinting rather than by plate wear, railroad tracking or blinding.
The use of sealant coating compositions to improve various aspects of lithographic printing plate performance is broadly old in the art. One prior suggestion is exemplarly disclosed in the 1956 and 1957 Bradstreet et al. U.S. Pat. Nos. 2,763,569 and 2,814,988 and another in the later 1966 Leonard U.S. Pat. No. 3,247,791. In the earlier Bradstreet patents, a method is disclosed for forming a white refractory type coating of extremely minute zirconia crystallites by the rapid thermal decomposition of minute droplets of a dilute solution of ammonium-zirconyl carbonate sprayed toward the surface of a zinc or aluminum lithographic plate substrate maintained at a temperature of 400.degree.-500.degree. F. The later Leonard patent disclosed a different chemical approach for a sealant surface utilizing phosphomolybdate coatings obtained by immersion of a plate in a molybdenum-phosphate solution followed by subsequent immersion in a sealing bath.
This invention may be briefly described, in its broad aspects, as an improved chrome surfaced photopolymer printing plate having a zirconium base, water insoluble, glasslike, thin and transparent compound selectively disposed intermediate the recessed portions of the chromium surface and an overlying coating of unexposed photopolymeric material. In a narrower aspect, the invention includes an improved lithographic photopolymer printing plate compositely formed of a close packed, cragged surfaced and highly porous layer of electrodeposited chromium on an aluminum alloy or steel base substrate, a thin layer of a water insoluble, zirconium base, transparent and glasslike compound selectively disposed in the nether recesses of said chromium layer and an overlying coating of unexposed photopolymeric material. In a still narrower aspect, the invention includes means for modifying the surface area and porosity characteristics of an electrodeposited chromium surface on a photopolymer lithographic printing plate. In still another aspect, the subject invention includes a method for selectively forming a relatively thin layer of a water insoluble, transparent and glasslike zirconium base compound on an electrodeposited surface layer of chromium on an aluminum alloy or steel base substrate photopolymer printing plate.
Among the advantages of the subject invention is the provision of a surface area reducing and porosity modifying compound for application to an electrodeposited chromium surface on a printing plate that is characterized by pronounced and prolonged water and fountain solution insolubility. Another advantage is the provision of a surface area and porosity modifying compound for chrome surfaced printing plates that does not deleteriously affect the adhesion of an overlying coating of photopolymer material thereto and which greatly minimizes background sensitivity and scumming on the non-image areas of a developed and finished printing plate. Another advantage of the subject invention is an improved surface area and porosity modifying compound that inhibits plate corrosion. A still further advantage of the subject invention is the provision of an improved surface area and porosity modifying coating compound for chrome surfaced lithographic photopolymer plates that is of pronounced hydrophobic character when coated with photopolymeric material, but which becomes possessed of pronounced hydrophilic characteristics in the non-image areas of the plate after exposure, development and finishing of the plate. A still further advantage is the provision of a directly chrome plated lithographic photopolymer printing plate that is attended by a marked reduction in tinting and scumming tendencies and an attendant significant increase in effective press life and print quality.
The primary object of this invention is the provision of an improved lithographic photopolymer printing plate.
Another primary object of this invention is the provision of an improved chrome plated lithographic photopolymer printing plate of the type wherein the chrome plated surface is of pronounced hydrophilic character after exposure, developing and finishing and is definitive of the non-image area in a press-ready plate.
Other objects and advantages of this invention will become apparent from the following portions of this specification and from the appended photomicrograph which depicts a presently preferred embodiment of a chromium plated printing plate having a surface area and porosity modifying coating incorporating the principles of this invention disposed thereon.